Automated guided vehicles, an unmanned vehicle capable of following an external guidance signal to deliver a unit load from destination to destination, has become a key element in the rapid growth in industrial automation of factories. The AGVs are particularly useful in transporting materials in warehouses, factories and other commercial and industrial settings. The AGV systems are capable of operating around the clock and have a tremendous impact on productivity.
The AGVs are very advantageous, primarily because of their adaptability and flexibility. Adaptability refers to the ability of AGV systems to be configured for the specific facility and material flow requirements of any particular operation at which the system is installed. Flexibility refers to the reduction in risks of obsolescence and increased utilization of the complete automation system. The specific areas of flexibility include, but are not limited to, volume flexibility, schedule flexibility, hardware flexibility, software flexibility, etc. Other advantages of AGV systems include real time control for responding to requests, floor space savings, ease of installation, reduction in labor costs, higher quality, reduced energy costs and many others.
The major classifications of AGVs include: driverless trains, such as tow vehicles; pallet vehicles; unit load vehicles, such as flat bed vehicles; fork-type vehicles; light weight limit load vehicles; and, special vehicles. The first five classifications are considered standard and have been developed for a broad range of applications. The special vehicles, on the other hand, are customized for specialized applications.
The average capacity of an AGV is between about one to three tons. Some of the heavy payload AGVs have a capacity of about 50 tons and are classified as high capacity, special vehicles. These vehicles are used to move items such as dies, coiled steel, sheet steel and rolls of paper which were previously handled and transported by overhead cranes.
The use of AGVs with a very high capacity of above about 100 tons, until now, has been hampered by design problems including those relating to the drive, steering and suspension. The very high capacity AGVs are particularly suitable for movement of extremely heavy rolls of material such as parent rolls of paper or coils of steel or aluminum, dies, ingots and casks. Also, AGVs for use with such a high capacity are often operated out of doors and must be able to handle the uneven roadway surfaces and the hostile weather conditions which are encountered. That is, the drive and steering mechanism must be able to operate at a reasonable speed while negotiating the turns, the rough road surfaces and the effects of weather, such as ice on the roadway. For example, this type of very high capacity AGV must particularly be suitable for handling large shipping containers at docks where they are loaded on or unloaded from ships and often stacked upon each other.
In the past, a variety of drive-steering geometries were available for steering and maneuvering. Examples include tricycle wheel geometry, dual triangle wheel geometry, differential drive wheel geometry and four-wheel steering geometry. Each of these steering systems, while being suitably operable for the smaller, prior art AGVs, has certain limitations and deficiencies when use in conjunction with the very high capacity AGVs capable of handling over 100 tons to which the present invention is related.
The tricycle wheel geometry comprises three wheels with two idle wheels at the rear of the vehicle and a drive wheel at the front center of the vehicle. This wheel configuration does not track well and the single drive wheel is likely to slip when the traction is poor due to floor conditions such as unevenness or wetness. Also, the vehicle using this drive configuration can be unstable unless the load is carefully positioned.
The dual tricycle wheel geometry is constructed of two tricycle wheel assemblies, as just described. The independent steering and drive wheels are disposed in the front and rear of the vehicle with the caster wheels on the sides of the vehicle. This wheel configuration can be difficult to steer because the caster wheels have a tendency to remain in a set position due to the weight of the load.
Differential drive wheel geometry includes two fixed, independently-driven drive wheels mounted in the vehicle's center towards the outside of the vehicle, with caster wheels at the corners to provide stability. The vehicle is steered by varying the speed and direction of each drive wheel. This system is not suitable for extremely heavy loads because only two drive wheels are not adequate to deliver enough drive force to the floor to properly drive a heavy loaded vehicle. Also, the uneven surfaces on which the very high capacity AGVs are operated, can cause one or both of the drive wheels to slip or be out of contact with the drive floor and thereby impede the control and operation of the AGV.
Four wheel steering geometry combines the steering and drive functions in each of the four wheels, wherein each of the wheels is located at a corner of the AGV. This is a typical arrangement for the normal heavy payload applications, i.e. up to about 50 tons. However, when this arrangement is adapted for an AGV with a load above about 100 tons and especially for use outdoors, it becomes deficient because one or more of the drive and steering wheels may not make adequate contact with the ground due to factors such as unevenness or slick road surfaces.
Another problem encountered in connection with the use of prior art AGVs for heavy loads relates to the suspension systems for the wheels and especially the drive wheels, each of which is independently connected to the vehicle for driving and steering. When a heavy load is loaded onto the AGV, by means such as a crane, special devices have to be incorporated into the system to absorb the shock of loading as well as the forces imposed on the suspension system by the heavy load during operation of the vehicle.